The apparent superluminal motion of the fine-structure components of 3C 120

The changes observed in the fine structure of the variable radio source 3C 120 are consistent with the model of ultrarelativistic electrons being injected into a magnetic flux tube that expands with distance from the nucleus of the galaxy. This model is invoked to interpret the observed peculiarities in the motion of the radio components: the apparent braking of the resolved components whenever new components develop, the decay of components at the end of their separation tracks, and the frequency-dependent behavior of the components' positions relative to the central source. The component located o''.005--0''.007 from the central source oscillates in position; it probably represents the knot in the large-scale structure of the 3C 120 jet closest to the center